engineering essay

Characterisation Of Afm Tip Engineering Essay

Published: 23, March 2015

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For this laboratory experiment there will be an investigation into the characteristics of the AFM tip and cantilever. There will be multiple measurements of a calibration grid taken using the AFM, varying the scan parameters and using image analysis techniques improve the image quality. The correction factor will thus be found. The SEM will be employed to determine the cantilever and the tip parameters. These measurements will then be compared with the specification sheet. The SEM will also be employed in the X-ray florescence of the cantilever and tip, to characterise the material of which it is manufactured. This will lead to being able to define its mass, spring constant and resonant frequency using the appropriate formulas.

The scanning electron microscope (SEM)

The first SEM image was obtained byÂ Max Knoll, who in 1935 obtained an image ofÂ silicon steelÂ showing electron channelling contrast [1]. The scanning Electron microscope is a type of microscope that uses a finely focused electron beam to generate a variety of signals across the surface of solid specimens. The accelerating voltage for the electrons is typically 0 to 30 KV. Higher voltage generates:

More energy

Penetrates the specimen further

Has a larger interaction volume

Lack of contrast

Can cause specimen damage

Scanning Electron Microscope overview

Figure 1

The sample is scanned point to point and at each point the signal collected is mapped onto the display. Scan time can vary e.g. 300ms to 10ms, slower scan speeds means that the beam dwells longer at each point (pixel) thus larger signal is acquired and also gives a better signal/noise ratio. Scanning is accomplished by means of two deflection coils at right angles to produce a raster scan. Each scan contains 1,000,000 points. The by-product of the electron interactions are:

For Backscattered electrons the solid state detector is widely used: Electron/hole pairs produced in the semi-conducting wafer normally recombine. A small potential difference across the wafer sweeps up the free charges (current). 3.8V is needed to generate each electron hole pair thus a 10Kev supply will produce 2600 electrons, which amounts a very small current. An amplifier is used to vary the intensity of the display on the CRT. The amplifier used is the Scintillator/photomultiplier Electron Detector which Converts: -Secondary electrons to photons, -photons to photo electrons and finally photoelectrons to electrons. The SEM resolution is primarily limited by the beam diameter (spot size) which is typically 5nm. The spot size also reduces the signal to noise ratio. The maximum magnification in the SEM = = 20,000. Some instruments may provide a higher magnification but no more detail [1].

The Atomic Force Microscope (AFM)

The Atomic Force Microscope is a scanning probe microscope with a very high resolution. The AFM raster scans over the surface of the sample using a cantilever with a sharp tip.

Figure 2

The general operation of the AFM:

The sample is scanned in the x,y direction under computer control sending a charge signal to the piezoelectric transducer.

The cantilever deflects due to atomic forces between the tip and sample.

The deflection is measured by the laser detector.

The measured force is compared with the pre-determined reference force to determine the error.

The negative feedback loop moves the sample in the z direction to eliminate the error.

The computer records the z movements as a function of x,y to construct a 3-D image.

The image produced relies heavily on the characteristics of the tip and the cantilever. Tip convolution occurs when the size of the tip is greater than or equal to the radius of curvature of the specimen being sampled and as a result the topographic image is not a true image (Figure 3). The spring constant of the cantilever also plays a major role in the image resolution as it needs to have a high resonant frequency so as it is immune to background noise and also soft enough not destroy the sample.

Figure 3

The AFM can be used in attractive (dipole-dipole) or repulsive (electron cloud) force modes as is illustrated in Figure 4. The repulsive force mode will give a better resolution but may damage the sample, while the opposite is true for the attractive force mode due to residue or liquid on the sample.

Figure 4

The X-Ray Florescence (XRF)

This is an analytical technique that does both quantitative and qualitative analysis of a sample being tested. An accelerating voltage is used to bombard the sample with electrons. Electrons are displaced and ejected from their orbit. The vacancies are filled with electrons from the outer shells which have greater potential. The electrons that fill the vacancies releases their excess energy in the form X-rays. Each element has its own characteristic peaks in the X-ray spectrum [3].

Figure 5

The SEM is used for this procedure generating an electron beam with a high accelerating voltage (several KV) depending on sample being tested. The X-rays emitted from the sample strike a solid state detector usually cooled with liquid nitrogen. The detector absorbs the X-rays by ionisation and thus converts the X-rays into electrical voltages to a proportional size. These voltages are then converted into the corresponding X-ray spectrum of counts vs. energy (KeV).

Measurements of calibration grid obtained via the AFM

Measurements of the cantilever obtained via the SEM

Measurements of the cantilever tip obtained via the SEM

Figure 10 Sketch of cantilever tip

Table 6

Table 7 Showing the slope of the tip in AFM and SEM

Conclusion

Analysis of AFM and measurements obtained

The results obtained were gained following all guidelines and procedures from manual. From the results of the AFM scan on the calibration grid the uncertainties were deduced by getting the mean, standard deviation, the standard error of the mean and thus the %uncertainty. All dimensions obtained while calibration grid was perpendicular in orientation can be viewed in table 1&2, while the parallel orientation can be seen in tables 3&4. The correction factor for both measurements does not vary a great deal. The AFM software proved difficult to obtain a reasonable tip angle. This may be due to the fact the height of the grid is 0.2Âµm and the length of the tip 28.52Âµm thus only the slope can only be measured at very top of the tip. This may lead to convolution as shown in figure 3.

Analysis of SEM/XRF and measurements obtained

The results from the SEM show a huge difference between the measurements in AFM and SEM slopes for the tip. The tip for the SEM has been sputter coated which could only lead to minor discrepancies. There was relatively large piece of dirt on the tip of the cantilever which also hindered the measurements, as can be seen in figure 6. The large discrepancies in the slope measurements cannot be accounted for in uncertainties. The tip measured in the SEM is not the same tip used to measure the calibration grid thus tip damage cannot be ruled out. The SEM software proved to be problematic as the scan had to be prompted to refresh every couple of seconds, making it difficult to observe any change in magnification of focus while preparing the image for analysis.

The results from the XRF proved the cantilever and tip was manufactured from silicon.

The spring constant calculated is 0.0387N/m Â± 1.76% which is inside the specification

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